1. Field of the Invention
This invention relates to flexible, open-celled, free-rise polyurethane ("PU") foams made by molding or by continuous casting methods. In accordance with the present invention, carbon dioxide--from sources other than the reaction of water and free isocyanate--is present in the gaseous state during the foam-forming reaction. The carbon dioxide contributes to the rise of the foam and to the lowering of density of the foam product.
2. Description of Related Art
Flexible PU foams, of both the conventional and high resilient types, are produced from formulations typically containing an isocyanate reactant, one or more blowing agents including water, a foam stabilizer, catalysts, and polyether polyols. High resilient ("HR") foams are those which exhibit a rapid recovery from extreme compression. As compared with conventional foams, HR foams exhibit a higher resilience (55% to 65% for HR, compared with 40% to 55% for conventional) and a higher modulus (2.2 to 2.7 for HR, compared with 1.8 to 2.3 for conventional), where the modulus is defined as the ratio of the 65% Indentation Force Deflection ("IFD") measurement to the 25% IFD measurement.
PU foams are formed by the effervescent action of a gas on the product of a polymerization reaction. Specifically, isocyanate groups react with water to generate carbon dioxide gas. This gas is dispersed and, to a large extent, retained during the subsequent polymerization--which involves the reaction of additional isocyanate with hydroxyl groups of the polyols. At the time the polymerization is essentially complete, the foam gels (i.e., becomes solid) and the cells are usually broken open by the heat of reaction and the pressure of trapped gases to produce the well-known open cell structure of flexible PU foams.
The suitability of a particular PU foam for any given end use is determined primarily by two critical physical properties: its density and its firmness.
The density of a foam is principally controlled by adjusting the proportion of water in the formulation, which in turn regulates CO.sub.2 generation or "blowing". However, it is well-known that the reaction of water with isocyanate not only yields CO.sub.2 but also produces urea linkages in the polymer which make the foam firmer, less elastic, and more brittle. The use of water as a blowing agent thus does not permit the variation of density independently of other properties. Moreover, the reaction of water with isocyanate is highly exothermic. The heat generated can cause undesirable effects ranging from internal degradation, e.g., scorch, to decomposition and even combustion of the foam during the curing phase of the reaction. Therefore, the maximum amount of water which can be used is limited by safety considerations as well as by the foam properties desired.
For years the problems associated with controlling density by addition of water have been circumvented by including in the formulations certain low-boiling, nonreactive liquids, e.g., methylene chloride or chlorofluorocarbons. These auxiliary blowing agents are converted to gases by the heat of the polymerization reactions. Since these gases are not incorporated into the polymer structure, they do not cause firming or embrittlement. Auxiliary blowing agents thus have enabled the lowering of density without an increase in firmness, resulting in soft, stable, resilient foams. However, the use of these agents is now seen as undesirable, due to concern about the health effects of exposure to methylene chloride vapors and due to concern about adverse environmental effects, specifically, depletion of the atmospheric ozone layer which may result from the release of chlorofluorocarbons into the atmosphere.
Various methods have been suggested in the art to eliminate or reduce the need for commercial auxiliary blowing agents. One such alternative proposes the use of carbon monoxide produced by the decomposition of formic acid as an auxiliary blowing agent. This method has not found acceptance because formic acid is highly corrosive and because the gaseous degradation product, carbon monoxide, is both highly toxic and flammable.
Another known method for lowering foam density involves pouring foam into molds or like containers, which are placed in a sealed chamber. Reduction of atmospheric pressure within the chamber during the rise of the foam produces a finished product of reduced density without the addition of gas-producing constituents. But such treatment is difficult to control and is not applicable to the majority of commercial manufacturing facilities, which produce continuous buns or blocks of flexible PU foam.
It has also been found that the presence of air and other dissolved gases in the foam-forming ingredients, before or during mixing, results in the formation of bubbles and unacceptable holes in the cured foam product. Technology does exist for producing stable emulsions of gases, including air, in rising foam. But this frothing method, in its present commercial form, requires the use of pressurized foaming equipment and is not suitable for the preparation of low density and/or soft foams.
Carbon dioxide is considered to be a desirable auxiliary blowing agent. When added to foam formulations, it does not react to produce urea structures or heat. Moreover, CO.sub.2 would not pose any problems of flammability and would decrease the level of toxic or possibly harmful effluents in the plant and atmosphere. But the simple addition of carbon dioxide in gaseous form at the mixing head is not effective. No measurable effect on either density or firmness is achieved.
Other methods have been proposed for introducing CO.sub.2 into the foam-forming reactants. U.S. Pat. No. 4,906,672 discloses a process for incorporating CO.sub.2 into the feed stream of one or more of the liquid reactant constituents of the foam formulation. Specifically, gaseous CO.sub.2 is injected under pressure into a feed stream at a distance from the mixer which will maximize the dissolution of CO.sub.2 in the feed stream. The gas is then held sufficiently strongly in solution so as not to be released until such time as it can be retained in the expanding reactant mass and can contribute to the cell structure of the final, stabilized foam product. The effect on the foam is a reduction of density with only a slight decrease in the firmness or load-bearing ability of the foam.
In U S. Pat. 4,284,728, the addition of carbon dioxide is proposed as a stabilizer, i.e., to reduce the reactivity of reactive amines used as cross-linking agents in the foam-forming reaction to produce HR foams. A blend of polyol and diamine cross-linking agent is treated with CO.sub.2, as by sparging, up to a concentration of 2.0 moles of CO.sub.2 per equivalent of cross-linking agent. But HR foams obtained using the CO.sub.2 treatment exhibit a higher density, rather than a lower density, than those made without CO.sub.2 treatment under comparable conditions.
EPO 145,250 suggests that adducts of CO.sub.2 can be prepared for use as additional blowing agents in PU foam manufacture. One such CO.sub.2 adduct is a salt produced from the reaction of CO.sub.2 with a water-soluble amine, in the presence of water and polyol. The adduct is then destabilized by reacting with tolylene diisocyanate ("TDI"), thereby releasing the CO.sub.2 as a gas in the foam-forming reaction mixture. This method, as disclosed, is limited to the mixing and dissolution of solid and gaseous CO.sub.2 in a pressurized vessel in a polyol that also contains water, trichlorofluoromethane, silicone, tin catalyst and an amine accelerator.
While the method of EPO 145,250 appears to permit the 10 reduction of water and isocyanate in the foam-forming reaction, the quality of the final foam product is not fully disclosed. (In one example, foam shrinkage was reported.) Moreover, it is common industrial practice to switch from production of one foam grade to another "on the fly", i.e., without interrupting the flow of materials to the mixing head. The EPO 145,250 method of incorporating the CO.sub.2 by dissolution in the polyol component --which also contains all of the other "low molecular weight fluids" that comprise the formulation--could therefore be impracticable under commercial operating conditions.
The use of a CO.sub.2 adduct is also disclosed in U.S. Pat. No. 4,735,970 in the preparation of rigid foams by the frothing process. The adduct is formed by the reaction of CO.sub.2 with specified amines containing at least one secondary amino group, no primary amino groups, and at least one primary or secondary hydroxyl group, provided there are not more primary hydroxyl groups than amino groups. The adduct optionally contains water. Although it is stated that the adduct can be employed in the manufacture of flexible and semi-rigid foams by block foaming and by the laminator process, no examples are provided other than frothing of rigid foams.
Finally, U.S. Pat. No. 4,500,656 discloses methods for making stable liquid CO.sub.2 adducts of low molecular weight amines and amino alcohols such as diethanolamine (DEOA), preferably in the presence of some water. Although the disclosure ostensibly is for the general use of these adducts in PU foams, the discussion and examples--based on the hydroxyl numbers of the polyols utilized--are directed only to rigid and semirigid foams, and to the frothing process. There are no examples of utility or benefits in the context of flexible PU foams.
The failure of U.S. Pat. No. 4,500,656 to address flexible PU foams is not surprising. Liquid CO.sub.2 : amine salts (e.g., DEOA:CO.sub.2) would have to be present in foam formulations in concentrations greater than 2 phr (parts by weight per hundred parts by weight of polyol) in order to bring about measurable density reductions in the final foam product. DEOA and similar materials have been used in some types of HR flexible foams -but only at very low concentrations, i.e., up to 2 phr. It is well-known in HR foam technology that greater concentrations of DEOA cause undesirable softening of foams. This softening can be mitigated by a number of modifications known to those familiar with the art, such as changing the polyols in the foam formulation, adding crosslinkers, increasing the TDI index, increasing the functionality of the isocyanate, or blending MDI (methylene diparaphenylene diisocyanate) with TDI. However, such remedies work only within relatively narrow limits. Moreover, DEOA, when present during the foaming reaction at levels higher than 2 phr, generally leads to instability which cannot be overcome by conventional means, i.e., by increasing catalyst levels. This has been a problem particularly in the production of lower density HR foams.